The regulation of cysteine biosynthesis in S. typhimurium involves a complex interacting system of feedback inhibition and repression. The control of gene expression is positive rather than negative as is the case in the well-defined lac operon. Regulatory elements include one or more of the biosynthetic enzymes, an unidentified regulatory gene product, cysteine itself, and several metabolic intermediates. By means of careful genetic and biochemical analyses we shall attempt to define the manner in which these many substances interact to control the rate of cysteine biosynthesis. Efforts will be made to isolate and characterize the as yet unidentified cysB gene regulatory product. Mutants in the structural gene for cysteine desulfhydrase will be searched for and if obtained will be used to study the different effects of sulfide and cysteine on repression. The enzyme serine transacetylase will be purified by a combination of antibody and affinity chromatography from wild-type and overproducing strains.